The present invention relates generally to diffractive imaging laser systems, and more specifically to an optical parametric amplifier system which is designed to act as an amplifying variable spatial bandpass filter.
Much of the early work in optical parametric processes concerned the upconversion of weak infared signals as well as some of the associated imaging properties. Parametric amplifiers are now commonly used as a means of amplifying optical waves. In these systems a coherent pump wave is made to interact with a nonlinear optical crystal to produce amplifiction at two other optical wavelengths.
More recently, interest has shifted to parametric down conversion with an emphasis on the nonclassical aspects of photon correlations, photon interferences, and the production of squeezed states. A parametric converter is a conventional optical device which converts an input optical signal at one frequency into an output signal at a different frequency. A parameter down-converter is a particular parametric converter in which the output signal has a frequency which is lower than that of the input signal.
The amplifying character of parametric down conversion in combination with the possibilities of low noise detection by squeezed states leads to some interesting imaging applications. However, there have not been any systems for diffractive imaging in parametric processes. The task of providing such a system is alleviated; to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 3,488,106 issued to Lohmann; PA0 U.S. Pat. No. 4,320,358 issued to Peterson et al; PA0 U.S. Pat. No. 3,432,768 issued to Garwin; PA0 U.S. Pat. No. 3,771,065 issued to Goldberg et al.; PA0 U.S. Pat. No. 3,417,346 issued to Yatsiu; PA0 U.S. Pat. No. 4,048,516 issued to Ammann; PA0 U.S. Pat. No. 4,315,224 issued to Ezekiel et al.; PA0 U.S. Pat. No. 4,318,057 issued to Buckwald et al.; PA0 U.S. Pat. No. 4,264,877 issued to Grischkowsky et al.; and PA0 U.S. Pat. No. 4,294,508 issued to Husbands.
The above-cited patents include references that disclose laser systems in which a pump frequency laser is combined with a signal frequency laser to create another beam. Yatsiv is concerned with a laser having Raman material for double quantum absorption and Buchwald et al. speak of a Raman laser which uses two optical pumping mechanisms. Ezekiel et al. use three pumping lasers on a molecular beam, but the second and third pumps are not at a point further down the beam from the first optical pumping source. Also, in the Ezekiel et al. reference, construction of two of the optical pumping beams are at the same frequency. Stimulated Raman Scattering (SRS) is disclosed in the Amann patent.
Grischkowsky et al. disclose a laser which may employ the Stimulated Raman Scattering (SRS) process. The patent shows embodiments with various plural pumping arrangements and discusses both a two photon process and a three photon process. The Husbands reference discloses an optical multiplexer which combines a plurality of laser beams. The Lohman patent discloses a spatial filtering system for performing differentiation.
While the above-cited references are instructive, there remains a need for frequency down-converting spatial band pass filters. The present invention is intended to satisfy that need.